A high density record such as a video disc of the type described in U.S. Pat. No. 3,842,194 issued to J. K. Clemens is used in the playback system of the variable capacitance type. In one configuration of the Clemens' system, information representative of recorded picture and sound is encoded in the form of a relief pattern in a relatively fine spiral groove on the surface of a disc record. For example, groove widths of approximately 2.6 micrometers and groove depths of about 0.5 micrometer may be used. During playback, capacitive variations between a conductive electrode on a stylus and a conductive property of the disc record are sensed to recover the prerecorded information.
ln accordance with the Clemens' format, the video information may be recorded as relatively short (e.g., 0.6-1.6 micrometers) relief variations along the length of the spiral groove. Illustratively, the method of recording may be of a type shown in U.S. Pat. No. 4,044,379 to J. B. Halter. Pursuant to the Halter method, an electromechanically-driven stylus (e.g., of diamond) having a triangular shape, responsive to a combined video and audio signal, records relatively short geometric variations, representative of the time variations of the signal, on a surface of a metal substrate. After the electromechanical recording operation, the recorded surface of the metal substrate has a relief pattern corresponding to that which is desired in the final record. In the replicating process, masters are made from the substrate. Molds are then made from the masters and stampers are made from the molds. The stampers are used in the process of pressing a vinyl record having the desired relief pattern.
During each of the above-identified record manufacturing processes, various kinds of defects can develop which may affect the record groove quality and which are difficult to detect in view of the fineness of the groove structure typically employed in a video disc (e.g., 10,000 groove convolutions per inch [4000 convolutions per cm]).
Because of the very small dimensions of the signals recorded on the video disc, very small defects can disturb the playback of a disc. Many defects which cause trouble in playback are difficult to see when viewed under normal lighting or when observed through a microscope.
Irregularities in the surface of a video disc contribute to various deficiencies in performance, such as stylus and disc wear, excessive noise in the audio channel derived by the playback apparatus, and excessive streaking in the color channels, more usually identified as the chroma channel. Some of these irregularities appear on the surface of the video disc in a form which has become known as "orange-peel." Techniques developed by H. N. Crooks, et al. for determining the orange-peel in video discs by optical means are disclosed in copending U.S. application Ser. No. 485,474, filed Apr. 15, 1983, now U.S. Pat. No. 4,541,716, issued Sept. 17, 1985, which patent is incorporated by reference herewith. See also copending application Ser. No. 307,021, now U.S. Pat. No. 4,481,616, filed Sept. 30, 1981 by J. E. Matey, now U.S. Pat. No. 4,481,616, issued Nov. 6, 1984, for a description of a method for determining topography variations in a video disc.
There is a need in the manufacture process of video discs to measure and quantify surface irregularities in order to identify deficient manufacturing operations and to provide quality control of video disc production.